The present invention relates to a method and arrangement in a frequency converter as described in the preambles of independent claims 1 and 4. In particular, the invention relates to the charging of a capacitor of an intermediate circuit of frequency converters with voltage intermediate circuits.
A frequency converter is a device used to generate an adjustable alternating voltage by using a constant-frequency voltage. The most typical application of frequency converters is controlling motors in such a manner that an adjustable voltage is generated from the alternating voltage of a supply network to control the motor. A frequency converter with a voltage intermediate circuit comprises a rectification unit, voltage intermediate circuit, and inverter unit. A task of the rectification unit is to provide direct voltage from the supplied alternating-current voltage to the voltage intermediate circuit, to which one or more capacitors are connected to store and equalize the direct voltage depending on the power of the device. The inverter unit connects the direct voltage of the voltage intermediate circuit to the load in such a manner that the direct voltage pulses generate the desired average alternating-current voltage or a corresponding adjustable property.
When switching a frequency converter on, the capacitor of the intermediate circuit should be charged before the actual use of the frequency converter. The charging requires a separate charging arrangement that limits the magnitude of the charging current. If the charging current were not limited, the current would increase in magnitude so much that the protective devices in the supply would switch the supply voltage off. Known solutions for charging the capacitor of an intermediate circuit include various charging resistance arrangements, in which the charging current is reduced by a series resistor that is bypassed with a switch after the charging of the intermediate circuit. Another known way is to use a half-controlled bridge circuit, in which the magnitude of the charging current can be controlled by altering the firing angle of the controlled components of the bridge circuit, which requires that the charging arrangement be synchronized with the supply voltage.
A drawback with both above-mentioned known solutions is that the charging arrangements should be dimensioned separately for each frequency converter power. In large MW-power frequency converters or system drives, the charging circuit also becomes quite large and expensive.